The present invention relates to a vacuum stabilizer and a method for the manufacture thereof.
Such stabilizer can be advantageously exploited for maintaining a satisfactory vacuum degree inside a thermally insulating jacket under vacuum, where also insulating materials of different type may optionally be present.
A vacuum jacket of this kind allows to eliminate or considerably reduce the heat exchange between the environment and an inner chamber, or piping, lying at a temperature different from room temperature, generally lower.
A few typical examples of these end-uses are the Dewar vessels, employed for the storage of cryogenic fluids, as well as the pipings for the transport of same fluids or the insulating panels of refrigerators and freezers; a very and interesting recent case is represented by the tankers containing liquid methane or the liquefied petroleum gases (LPG).
It is known, in the current practice, that a good thermal insulation may be realized by means of a vacuum jacket, which may optionally contain insulating materials like glass wool, colloidal silica, perlite or organic polymers, in the form of foams (for instance open cell rigid polyurethanes or expanded phenolic resins) or in the form of a multilayer, particularly a polyolefinic multilayer mode from polypropylene, polyethylene, olefinic copolymers and so on).
It is known too that the vacuum realized in such a jacket, during its preparation, tends to degrade with the time, because of the degassing of the walls of the jacket, because of a gas infiltration (e.g. CO, CO.sub.2, O.sub.2, H.sub.2, H.sub.2 O, N.sub.2 and so on), because of the possible leaks of the walls or finally, in the case of a wall made from a plastic material, because of a permeation of gases through the walls, particularly the atmospheric gases.
Such a drawback is overcome, in the practice, by introducing one or more materials allowing to fix said gases and the vacuum is normally maintained by resorting to gas sorbing materials, located in the jacket.
These gas sorbing materials can operate according to the physical sorption principle, like for instance in the case of the zeolites and of other molecular sieves, of active carbon and so on; a drawback of these physical sorption agents, however, resides in a reversible sorption of the gases, as a function of temperature, and therefore they are not completely suitable for the purposes of the thermal insulation. Better results, to this purpose, are reached by employing materials which can irreversibly adsorb and fix the gases, according to a chemical reaction, like for instance the drying agents, in the case of water vapor, or the so-called getter materials, in the case of gases like CO, CO.sub.2, O.sub.2, N.sub.2 and so on or, again, water vapor.
In the following, when not better specified, the drying agents and the getter materials will be generally indicated as chemical sorption agents or simply "sorption agents"; this chemical technique is well known from the state of the art, for instance from the international publication WO 93/25,843 in the name of the Application and published on Dec. 23, 1993, after the priority date of the present application.
The document, cited as a reference with respect to the physico-chemical aspects of the sorption of gases and vapours, describes the combined use of a drying agent, selected from barium oxide, strontium oxide, phosphorus oxide and mixtures thereof, and of a non-evaporable getter material essentially consisting of an alloy containing barium and lithium, in particular an alloy having the raw formula BaLi.sub.4 ; according to said document, it is optionally possible to introduce into the jacket also the oxide of a noble metal, preferably palladium, allowing to convert the last traces of hydrogen into water, which is then chemically fixed by the drying agent. As to the purpose, the use of silver and/or ruthenium oxide give rise to equally satisfactory results as the ones coming from the use of Pd oxide.
Said PCT document was teaching too to introduce the drying agent and the getter material into a housing subdivided into two separate zones by means of a porous septum; the drying agent is lying in the outer zone and the getter material in the inner zone, which is only indirectly in communication with the jacket empty space just because of the existence of said porous (gas permeable) septum and the said outer zone containing the drying agent.
The method described in said PCT application is already representing an improvement, with respect to the known technique, as to the physico-chemical aspects of a long lasting vacuum maintenance. Said method, however, does not eliminate a drawback which was normally present in the field of the sorption materials; these materials in fact, because of their peculiar prefixed task, have to be highly active with respect to the gases to be absorbed. On the other side, the reactivity of the drying agents and/or getter materials should be inhibited for a time of a few days or even months, when they are manufactured, as usual, by means of a process different from the manufacture of the end-use jackets.
This drawback can be in a few cases by-passed by using materials showing a low reactivity in the air, before the activating thermal treatment, to be performed only when the getter is already assembled in its final working position and when the jacket is already evacuated and sealed. The thermal treatment, however, generally requires temperatures of at least 200.degree. C., not always compatible with the material of the jacket's walls, as in the case for instance of the panels made from plastic materials employed for the thermal insulation of refrigerators, freezers and tankers described for instance in U.S. Pat. Nos. 5,018,328 and 5,091,233; other solutions of the problem, providing no thermal activation, were proposed in other different patents.
U.S. Pat. No. 3,114,469, for instance, describes a vacuum jacket containing, fastened to the outer wall, a capsule made from a material which can undergo a distortion without loosing its gas-impermeability; said capsule contains a housing made from a material, for instance glass, which can be easily broken consequently to the distortion of the outer envelope, and the housing contains in its turn the getter material. Such a solution of the problem is theoretically effective but it is difficult (and expensive) to carry out a practical realization, on an industrial level, and anyhow the welding between said outer envelope and the outer wall of the vacuum jacket is representing the critical point of the structure, corresponding to a not neglectable danger of breakage.
U.S. Pat. No. 4,668,551 describes a thermal insulation unity in the form of an envelope made of plastics, containing an insulating material of the type hereinabove, some activated carbon and a gas impermeable bag containing a zeolite powder previously conditioned in an atmosphere of a gas which cannot be adsorbed by the zeolite, like hydrogen, helium, neon, alcohol vapours or vapor of hydrocarbons having at least 3 C atoms.
The zeolite containing bag is then opened during the sealing of the thermal insulation unit, occurring in an evacuated room. The method described in this patent is however showing a few drawbacks. First of all the employed zeolites are poorly effective as to the maintenance of the vacuum degree; furthermore it is required a peculiar process step for gas-conditioning the Zeolites and finally it is necessary to add to the thermal insulation unit, besides the zeolite containing bag, also an activated carbon, in order to adsorb the gases (generally organic gases) which are not adsorbed by the same zeolite.
At last, U.S. Pat. No. 5,191,980, granted to the Applicant, describes a getter housing sealed by a film of thermoretractable material; the getter material gets into contact with the inner space of the vacuum jacket when the jacket is already sealed, because of a heat treatment allowing the thermo-retractable film to shrink and to get broken. Also in this use, however, the heat treatment required by the film opening may be incompatible with the jacket's walls, when made from plastics.
It is thus an object of the present invention to provide a method for the stabilization of the vacuum level inside an insulating vacuum jacket which do not suffer from the drawbacks of the known technique.
A second object of the present invention is to provide a combination of gas-sorbing materials which do not require any thermal activation during the manufacture of the relevant jackets.
A further object of the present invention is to provide a vacuum stabilizing device, or, shortly, vacuum stabilizer, containing said combination of sorption agents, preventing the deterioration of the same sorption agents, even for a period of months, allowing a rapid and simple activation of the same agents at the time of introduction of said sorption agents into the end-use vacuum jackets.
A still further object of the present invention is to provide a method for the manufacture of said vacuum stabilizer.